Physical questions

Robert G. Brown rgb at phy.duke.edu
Wed Jan 22 08:32:37 EST 2003


On Tue, 21 Jan 2003, Jim Lux wrote:

> > SO I wouldn't worry.  I don't worry much about cell phones, either.
> > Microwave ovens, I worry about.  TV sets and CRT's in general I worry
> > about (they produce soft X rays and other ionizing radiation with some
> > probability, especially close to the tube).
> I wouldn't sweat the TV set/CRT... Color TVs used to emit Xrays back when
> they used vacuum tube rectifiers and before they used lead glass in the
> rectifier tube envelope (Color TVs running at anode potentials of 25 kV or
> so, compared to the 10-15 kV for a B&W set..).. The front face of a CRT is
> lead glass and quite thick, just for this reason, as well (since what makes
> that spot is the electron beam stopping at the front...) (I seem to recall
> reading that there is 2-5 kilos of lead in the average monitor dumped in the
> landfills...making it a significant hazmat problem)

The lead alone is an excellent reason to use flat panel monitors,
although they may well have heavy metal problems of their own (e.g.
arsenic in their silicon).  Still, arsenic is less toxic than lead and
there is a lot less of it.

However, lead or not, I'd want to see a spectral analysis of the
downstream radiation, sampled over weeks to months, per monitor, before
I stop "worrying" about a given CRT or TV, as what the FCC or OSHA
considers "safe" may not be what I consider "safe".  I have spent
(many!) years of my life sitting within two meters of the front face of
one monitor or another over the last couple of decades, and ionizing
radiation is "different" from RF.  The risk is quantum mechanical --
either a given photon hits something it shouldn't and causes it to break
all at once or it doesn't, on a purely statistical, quantized basis.  It
isn't like being in a field of RF radiation and gradually absorbing
enough energy to heat and "burn" tissue and break bonds.  Even very weak
sources of ionizing radiation will cause cellular damage at some
probabilistic rate -- the only question is whether one is "unlucky" and
one of the transitions initiates cancer or damages retinal cells (one at
a time and very slowly) that don't repair or replace themselves at a
rate so that over twenty years of gradual exposure vision is
significantly affected or melanomas start popping up on your face.  The
fact that your cancer was unlikely doesn't leave you any less dead.

A cynical person is also just naturally curious about multiple
scattering and equipment malfunction and whether or not design/safety
guidelines adequately protect against the unexpected as well as expected
modes of operation.  Multiple scattering can deflect an electron so that
its path isn't what you think that it is so it produces its potentially
ionizing photons around the periphery of the screen, possibly unblocked
by the lead (?).  Low probability I'm sure FOR CORRECTLY FUNCTIONING
EQUIPMENT.  Equipment malfunction, however, is subject to the laws of
Murph.  A small resistor burns out, and suddenly every electron beam
sweep is scattering along pathways the manufacturer never intended or
imagined.  Another burns out, and suddenly the beam current is increased
by some unexpected factor.  A capacitor blows, and a single inductive
surge delivers a voltage ten times larger than expected to the electron
beam, for a very small period of time, showering one is a burst of
middling hard x-rays.

Do monitor safety guidelines protect against all of these things (and
ones I haven't imagined)?  Maybe, probably, mostly, but Murphy says
there are almost certainly surprises both in the steady state and in the
unpredictable catastrophic state of operation.  I try to sit back a
couple of meters from any CRT anyway, as I'm indeed a low cynical type.

CRT's haven't been in ubiquitous use for much more than 20 years at this
point (PC revolution being from maybe 1982 to 1986 or 1987, by which
point nearly every office desk had them).  Long term health problems
caused by all those CRT's, leaded screens or not, may not surface for
another 20 years, and engineering guidelines in the meantime are
dictated by what what people IMAGINE to be an adequately safe exposure
level.  The same people who did nothing when the Ctrl and Alt shifts
were moved into carpal tunnel position on a keyboard, the people who
still allow monitors to be built that run at 60 Hz scan frequencies, the
very same people who regulate the amount of mercury and sulphur
permitted in coal burning power plant smoke, the people responsible for
keeping things like snake-head fish far from our shores, the ones who
regulate the industrial exploitation of our oil fields and forests.  Our
government and industry leaders, who clearly have our best interests at
heart, always.

So sure, CRTs are safe.  I'm certain of it.

> > Computer engineer student types.  I don't think that they equipment is
> > particularly cheap or DIY, though...
> 
> Pshaw... high school students build cyclotrons out of spare parts in the
> garage....

Perhaps from a very advanced high school, but hey, cyclotrons are
relatively easy.  A big magnet made out of wire scavenged from the
alternator wrapped around a core made of the melted-down engine block,
an HVRF oscillator made out of parts from the old TV, a vacuum pump made
out of an old washer and bicycle pump -- piece of cake.  Of course the
high vacuum beam pipes and quadrupole magnetic lenses might present a
bit of a problem, but anyone who has read Heinlein realizes that really
bright kids can easily build nuclear rockets and space suits.  Coming up
with enough lead to build a wall around your apparatus so you can
actually operate your homebrew cyclotron without giving your little
brother cancer might be dicey, though.  Presuming you WANT to avoid
giving your little brother cancer...:-).

So sure, for these high school students building a decent oscilloscope
out of an old TV or PC monitor, building a multichannel analyzer,
building the requisite RF tuning and amplification circuitry so that it
will work across a half dozen decades of frequencies would be trivial.
Sorry I ever doubted it:-)

Gotta go now.  Time to turn my gas grill into something that can melt
iron in a flowerpot crucible so my teenager can start on his science
fair project...;-)

    rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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